diff --git a/reviewboard/extensions/hooks.py b/reviewboard/extensions/hooks.py
--- a/reviewboard/extensions/hooks.py
+++ b/reviewboard/extensions/hooks.py
@@ -55,9 +55,9 @@ class ActionHook(ExtensionHook):
         This should be a dict with the following keys:
 
            * `id`:           The ID of this action (optional).
-           * `image`:        The path to the image used for the icon.
-           * `image_width`:  The width of the image.
-           * `image_height`: The height of the image.
+           * `image`:        The path to the image used for the icon (optional).
+           * `image_width`:  The width of the image (optional).
+           * `image_height`: The height of the image (optional).
            * `label`:        The label for the action.
            * `url`:          The URI to invoke when the action is clicked.
                              This could be a javascript: URI.